1. Field of the Invention
The present invention relates to an exhaust gas purifier and a method of purifying exhaust gas for a hybrid vehicle which is driven using power from an internal combustion engine and power from an auxiliary power source.
2. Description of the Related Art
Demands have been made in recent years to reduce the fuel injection amount of internal combustion engines and the amount of exhaust gas discharged therefrom. To meet such demands, hybrid vehicles have been developed having two power sources, an internal combustion engine and an electric motor.
For example, one such hybrid vehicle is known which has an internal combustion engine, a generator operated by power from the internal combustion engine, a battery for storing electric power generated by the generator, an electric motor operating through the electric power generated by the generator and/or the electric power stored in the battery, wheels mechanically coupled to a rotational shaft of the electric motor, and a power distribution mechanism for distributing the power from the internal combustion engine to the generator and the rotational shaft of the electric motor.
If the load applied to this hybrid vehicle is low, for example, at the time of take-off or when travelling at a low speed, the engine is prevented from operating and the electric power from the battery is applied to the electric motor. The electric motor causes its rotational shaft to rotate by means of the electric power from the battery.
In this case, the rotational shaft of the electric motor rotates by power generated by itself and the rotational torque of the rotational shaft is transmitted to the wheels. As a result the hybrid vehicle travels only by the power from the electric motor which operates by the power from the battery.
If the load applied to the hybrid vehicle is intermediate, for example, when travelling at a normal speed, the engine is operated and the power distribution mechanism distributes the power from the internal combustion engine to the generator and the rotational shaft of the electric motor. The generator generates electricity using the power distributed by the power distribution mechanism. The electric power generated by the generator is applied to the electric motor. The electric motor causes its rotational shaft to rotate by the electric power from the generator.
In this case, the rotational shaft of the electric motor rotates by the sum of the power generated by the electric motor itself and the power of the engine distributed from the power distribution mechanism, and the rotational torque of the rotational shaft is transmitted to the wheels. As a result the hybrid vehicle travels by the power from the engine and the power from the electric motor which operates by the electric power generated by means of the power from the engine.
If the load applied to the hybrid vehicle is high, for example, during acceleration, the engine is operated and the power distribution mechanism distributes the power from the engine to the generator and the rotational shaft of the electric motor. The generator generates electricity using the power distributed from the power distribution mechanism. The electric power generated by the generator is applied to the electric motor together with the electric power from the battery. The electric motor causes its rotational shaft to rotate by the sum of the electric power from the generator and the electric power from the battery.
In this case, the rotational shaft of the electric motor rotates by the sum of the power generated by the electric motor itself and the power distributed by the power distribution mechanism, and the rotational torque of the rotational shaft is transmitted to the wheels. As a result the hybrid vehicle travels by the electric power generated by means of the power from the engine, the power from the electric motor operating by the electric power from the battery, and the power from the engine.
If the hybrid vehicle is being decelerated or braked, power regeneration is carried out making use of the fact that the rotational torque of the wheels is transmitted to the rotational shaft of the electric motor. That is, since the wheels are mechanically coupled to the rotational shaft of the electric motor and the rotational torque of the wheels is transmitted to the rotational shaft of the electric motor when the vehicle is being decelerated or braked, the aforementioned hybrid vehicle is able to carry out so-called power regeneration wherein the electric motor is operated as a generator to convert the kinetic energy transmitted to the rotational shaft of the electric motor from the wheels into electric energy. The electric power regenerated by the electric motor is accumulated in the battery.
If it becomes necessary to charge the battery or warm up the engine in the aforementioned hybrid vehicle when the engine is to be stopped from operating, the engine is started and warmed up, and the power from the engine is transmitted to the generator through the power distribution mechanism so that the generator generates electricity.
Such a hybrid vehicle enables the engine to operate effectively and makes it possible to reduce the fuel consumption rate.
On the other hand, as for an internal combustion engine installed in a motor vehicle, it is also important to purify noxious gas components contained in exhaust gas such as hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxides (NO.sub.x).
To meet such a demand, there has been proposed an exhaust gas purifier with an exhaust gas purification catalyst such as a three-way catalyst, an oxidation catalyst, an NO.sub.x occlusion-reduction type catalyst and an NO.sub.x selective reduction type catalyst, and with an adsorbent which adsorbs unburnt fuel components in exhaust gas when it is at a temperature lower than a predetermined temperature range and which ejects the adsorbed unburnt fuel components when it is heated up to a temperature within the predetermined temperature range.
The exhaust gas purification catalyst mentioned above is activated at a temperature equal to or higher than a predetermined activation temperature (e.g. 300 to 500.degree. C.) and can purify the noxious gas components in the exhaust gas flowing into the catalyst when its air-fuel ratio is within a desired range (a catalyst purification window).
The aforementioned adsorbent is made, for example, from a porous material mainly containing zeolite. If such an adsorbent is at a temperature lower than a temperature where unburnt fuel components start to gasify, the unburnt fuel components in their liquid state are trapped in pores. If the adsorbent is heated up and reaches or exceeds a temperature where the unburnt fuel components start to gasify, the unburnt fuel components trapped in the pores gasify and are desorbed from the adsorbent.
In the exhaust gas purifier having such a construction, when the exhaust gas purification catalyst is in its non-activated state, for example, during the cold-starting of the engine, the unburnt fuel components in exhaust gas are adsorbed to the adsorbent without being discharged into the atmosphere.
If the adsorbent is heated up and reaches or exceeds the temperature where the unburnt fuel components start to gasify, the unburnt fuel components adsorbed to the adsorbent start to be desorbed. At this moment, since at least a portion of the exhaust gas purification catalyst (e.g. an inlet of the exhaust gas purification catalyst) is in its activated state, the unburnt fuel components desorbed from the adsorbent are purified by the exhaust gas purification catalyst together with the unburnt fuel components contained in exhaust gas.
In the exhaust gas purifier as mentioned above, it is considered that a large amount of high-temperature exhaust gas flows into the adsorbent, for example, when the engine is operated at a high load. In such a case, there is a concern that the unburnt fuel components adsorbed to the adsorbent might be desorbed at a time and that the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification catalyst might deviate from the catalyst purification window.
To solve such a problem, there is known an exhaust gas purifier for an internal combustion engine as disclosed in Japanese Patent Application Laid-Open No. HEI 10-61426. This exhaust gas purifier has an adsorbent and an exhaust gas purification catalyst disposed in an exhaust passage of the engine, the exhaust gas purification catalyst storing the oxygen in exhaust gas when the air-fuel ratio of the exhaust gas is higher than the stoichiometric air-fuel ratio and ejecting the stored oxygen when the air-fuel ratio of the exhaust gas becomes equal to or lower than the stoichiometric air-fuel ratio. The exhaust gas purifier is designed to operate the engine at a lean air-fuel ratio immediately after completion of the starting thereof.
In the exhaust gas purifier having such a construction, the engine is operated at a lean air-fuel ratio immediately after completion of the starting thereof so that oxygen is stored in the exhaust gas purification catalyst prior to desorption of the unburnt fuel components from the adsorbent and that the exhaust gas purification catalyst ejects oxygen upon desorption of the unburnt fuel components from the adsorbent. In this manner the exhaust gas purification catalyst is intended to converge the air-fuel ratio of exhaust gas into the catalyst purification window.
When the engine is operated at a lean air-fuel ratio, the power that can be outputted from the engine decreases compared with the operation in the neighborhood of the stoichiometric air-fuel ratio. For this reason there is a concern that the engine might not be able to output a power required by the driver and that the driveability might deteriorate.